A capacitive microphone typically includes a diaphragm having an electrode attached to a flexible member and a backplate parallel to the flexible member attached to another electrode. The backplate is relatively rigid and typically includes a plurality of holes to allow air to move between the backplate and the flexible member. The backplate and flexible member form the parallel plates of a capacitor. Acoustic pressure on the diaphragm causes it to deflect which changes the capacitance of the capacitor. The change in capacitance is processed by electronic circuitry to provide an electrical signal that corresponds to the change.
Microelectronic mechanical devices (MEMS), including miniature microphones, are fabricated with techniques commonly used for making integrated circuits. Potential uses for MEMS microphones include microphones for hearing aids and mobile telephones, and pressure sensors for vehicles.
Many available MEMS microphones involve a complex fabrication process that includes numerous masking and etching steps. As the complexity of the fabrication process increases there is a greater risk of the devices failing the testing process and being unusable.
Applicant has proposed a number of methods for the fabrication of pressure sensors, such as silicon microphones. For example, International Publication WO2004105428 describes a silicon microphone of the above type that includes a flexible diaphragm that extends over an aperture. A backplate is also provided that combines with the flexible diaphragm to form the parallel plates of a capacitor for the microphone. However, this and many of the prior art examples are so-called “top-side” application sensors. That is, in use the sensor is packaged in a device, for example a mobile telephone, such that an acoustic signal travels through a hole in the device and is indirectly received by the sensor. This arrangement will be described in further detail below.